The present invention relates to liners which are used to form an impermeable barrier. In particular, the invention relates to an improved liner which is capable of being used on steep slopes. Liners for which the present invention has particular application are those which incorporate a layer of swellable clay, such as bentonite.
The present invention is in the nature of an improvement to liners like the one shown in U.S. Pat. No. 4,501,788, in which a layer of granular bentonite is enclosed between two flexible sheets. In such liners, a water-soluble adhesive is used to attach the bentonite particles to a geotextile. The geotextile used will depend upon the application, but are preferably comprised of at least one woven polypropylene geotextile. Other fabrics include non-woven fleece-like geotextiles and lightweight polyester net-like scrims.
Bentonite clay upon hydration tends to expand and become a highly impervious material. However, like most other clays, exposure of bentonite to water causes it to become very slick, greatly reducing the ability of the clay to withstand shear stresses. As a result, use of clay liners on slopes must be restricted to those having relatively shallow angles.
Some manufacturers of clay liners have attempted to create shear resistance in clay liners by needlepunching connections between fabric layers through the bentonite layer. However, these systems have two distinct disadvantages. First, the strength of the connection formed by needlepunching fibers is low. In some cases, the expansion force of the hydrated bentonite is sufficient to break the needlepunched connections. Second, needlepunched fibers form a path for capillary migration of water through the bentonite layer.
Manufacturers of needlepunched liners have attempted to rely upon needlepunched connections to retain the bentonite between the fabric layers in an effort to avoid the use of adhesive bonding of the bentonite to a fabric layer. Such heavy reliance upon the needlepunched connections for both bentonite retention and shear resistance has resulted in problems with both delivery and performance of such products. Needlepunching is generally inadequate as a sole means for retaining bentonite between two layers of fabric. The bentonite tends to migrate laterally unless it is held in place by means such as adhesive bonding. The integrity of the needlepunched connections is further compromised by the flexible nature of bentonite liners. It is an important feature of such liners that they be capable of being rolled up into a carpet-like form, so that installation can take place in an expedited manner. The stresses introduced to connections between fabric layers as a result of the rolling up of the liner during the manufacturing process, results in significant stresses upon the interfabric connections. After a needlepunched liner has been rolled and unrolled, the needlepunched connections may not have sufficient strength to resist the expansion of the bentonite layer, or to withstand shear stress on a slope.
Bentonite-based liners have found significant acceptance for use in preventing groundwater contamination. Bentonite liners have been used successfully in hazardous waste and municipal solid waste landfills. An important consideration in the construction of such landfills is the slope angle of the excavation. For a given area, significantly greater landfill volume can be achieved by the use of steeper slopes. The use of steep slopes allows more efficient use of a particular landfill site.
Similarly, clay liners are often used in tank farm applications in which berms or excavations are formed with sloping sides. Steeper slopes in tank farm applications allow more efficient use of real estate.
Accordingly, it is an object of the present invention to provide an improved bentonite liner which may be used to form an impervious barrier along a relatively steep slope.
Another object of the present invention is to provide a bentonite liner which does not allow for the migration of liquid through the liner.
It is a further object of the present invention to provide a liner which has significantly improved resistance to shear stresses.
These and other objects and advantages of the invention are achieved with a liner which is comprised of a layer of bentonite granules adhered to a woven polypropylene geotextile. A second woven polypropylene geotextile is placed on or adhered to the bentonite layer to form a sandwich like construction comprised of two geotextiles with a layer of bentonite therebetween. Rows of stitches are used to connect the two geotextiles. The stitching is comprised of a polymeric thread, preferably a coated multifilament thread comprised of high density polyethylene filaments. The thread is applied to the liner in a two-thread chainstitch at a rate of approximately three stitches per inch. The lateral spacing of the rows will depend upon the degree of shear resistance required by a particular application. However, it is preferable that the rows be spaced approximately two inches apart. The rows should extend along the length of the liner, rather than in the direction of the width of the liner.
The objects and advantages of the invention will become more apparent upon a reading of the following specification, read in conjunction with the accompanying drawings: